The reassortant virus containing the surface antigens of the A/Udorn/72 (H3N2) human virus and the "internal" six genes from an attenuated avian virus was evaluated for virulence in squirrel monkeys. The reassortant virus replicated to the same level as its avian parent and was fully attenuated in monkeys. These results indicated that the internal genes of the avian influenza A virus were the primary determinants of attenuation for the monkey. Squirrel monkeys infected with the reassortant virus were resistant to wild type virus challenge. This represents a new method for the production of a live influenza A vaccine virus in which attenuating avian genes are transferred to a human virus. The resulting reassortant virus is attenuated but immunogenic in primates (and, hopefully, man).